1. Field of the Invention
The present invention relates to an image reading device utilizing a solid-state image pick up element as a charge-coupled device (CCD) or an MOS image sensor.
2. Description of the Prior Art
An image, for example of an original document, is focused on a light receiving surface of a solid-state image pick up element through a projection optical system composed of a lens, in combination, if necessary, with mirrors, filters, etc. Consequently the light receiving surface of said solid-state image pick up element has to be precisely set on the optimum image plane of said optical system.
The photoelectric converting part of the solid-state image pick up element is usually sealed hermetically in a package similar to that employed in the integrated circuits, but the connecting terminals for supplying the photoelectric converting part with necessary signals and for extracting output signals from the image pick up element are exposed to the exterior of said package, and such solid-state image pick up element has often been fixed by soldering such connecting terminals directly on a printed circuit board.
However, such solid-state image pick up element is merely removably placed on the printed circuit board before such soldering, so that it frequently happens that the image pick up element is fixed by the soldering in an inappropriate position, lifted at one or both ends thereof, or that the printed circuit board is fixed in a bent state, because of the insufficient flatness of the board. Consequently, in order to set the light receiving surface of the solid-state image pick up element at the optimum image plane, there have been required, upon fixation of the printed circuit board to a substrate, an adjustment for displacing the solid-state image pick up element in the photo-axial direction, and an angular adjustment for maintaining the light receiving surface of the solid-state image pick up element parallel to the optimum image plane. There have therefore been required a complicated mechanism for adjustment and a cumbersome adjusting procedure.
As an example of prior technologies, the Japanese Patent Laid-open No. 102159/1981 discloses a method of placing an elastic member between the substrate and the printed circuit board and maintaining the light receiving surface of the solid-state image pick up element in pressure contact with a reference plane of a transparent glass plate provided on said substrate utilizing elasticity of the elastic member. This method reduces the influence of the fixing state of the solid-state image pick up element on the printed circuit board and of the bending of the printed circuit board since the light receiving surface of the soild-state image pick up element is maintained in direct contact with the reference plane of the substrate, but it is also associated with certain drawbacks such as lowered reliability and frequent destruction of the image sensor since the light receiving surface thereof is subjected to a strong force.